ARTICLE IN PRESS

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Available

online

at

ScienceDirect

www.sciencedirect.com

Modern

and

contemporary

art

An

insight

into

the

mechanical

properties

of

selected

commercial

oil

and

alkyd

paint

ﬁlms

containing

cobalt

blue

Laura

Fuster-López

a,∗

_,

_Francesca

_Caterina

_Izzo

b

_,

_Valentina

_Damato

b

_,

Dolores

J. Yusà-Marco

a

_,

_Elisabetta

_Zendri

b

a_Universitat_Politècnica_de_València,_Instituto_{Universitario}_de_{Restauración}_del_Patrimonio,_Camino_de_Vera_s/n,₄₆₀₂₂_Valencia,_Spain b_Ca’_Foscari_University_of_Venice,_Department_of_{Environmental}_Sciences,_Informatics_and_Statistics,_Via_Torino_155/b,_Venice,_Italy

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received6February2018 Receivedinrevisedform 18December2018 Accepted18December2018 Availableonlinexxx Keywords: Modernoilpaints Commercialformulations Cobaltblue Driers Mechanicalproperties GC-MS

a

b

s

t

r

a

c

t

Pigmentsandbindersareknowntoinfluencedryingandageingmechanismsofpaintfilms.However,the studyofthelong-termmechanicalbehaviourofpaintfilmsanditsrelationshipwithpaintdegradation andconservationstillneedsfurtherinvestigation.Thispaperpresentsaninsightintotheroleofcobalt bluepigmentinthemechanicalpropertiesofsomemodernpaintfilmsbystudyingselectedcommercial oilandalkydpaintfilmscontainingcobaltblue.Inaddition,severalrepresentativemock-upscontaining cobaltbluemixedwithdryingandsemi-dryingoilsandsomecommonadditiveswerealsotestedfor comparisonpurposes.Opticalmicroscopy,SEM-EDX,XRF,VIS-reflectancespectroscopy,FTIR-ATRand GC-MSanalysiswerecarriedouttoidentifypigments,binders,additivesandfillers.Uniaxialtensile measurementswereruntotestthemechanicalperformanceofthestudiedpaintfilms.Thedifferencesin themechanicalbehaviourledtotheevaluationofthediscrepanciesfoundinthechemicalandphysical propertiesofthedifferentformulationsstudied.

1. Cobaltblue 1.1. Introduction

Thepresenceofwrinkles,cracks,ﬂakinganddelaminationis usually registered in modern and contemporary oil and alkyd paintings. Theyareusually theconsequence ofcomplex failure mechanismsasaresultofchemicaldegradationprocesses[1–6] andchangesinthemechanicalanddimensionalpropertiesofthe paintﬁlmovertimeandwheretheenvironmentdoesnot neces-sarilyplayaparticularrole[7–10].

Previousstudieshaveshownacorrelationbetweenthe differ-entcomposition,dryingandagingoflipidicbindingmediaandthe changesinthemechanicalpropertiesinpaintfilmsovertime.The mechanicalpropertiesmayalsobemodified bytheionicbonds formedbypigmentswiththecross-linkednetworkofthemedium. Thesechangesarerelatedtotheon-goingchemicalprocessesthat oilpaintfilmsexperience.Ingeneral,oilpaintfilmsbecomestiffer over time whichis an indicationofthe progressiveincrease of modulusofelasticity(E)andultimatetensilestrength(UTS),anda

∗ Correspondingauthor.

E-mailaddress:laufuslo@crbc.upv.es(L.Fuster-López).

decreaseinthestraintofailureor,inotherwords,withadecreasing abilitytoelongatebeforebreaking[11–13].Pigmentsgovernsuch changesandtheirmagnitudebutalsodeterminehowearlysuch changeswilltakeplaceinthedryingprocessofagivenpaintfilm [14,15].Forexample,somepigmentscanreactwiththefreefatty acidsandvariouscompoundsformedduringoxidation andmay inhibitthediffusionofoxygeninthepaintfilmaswellasthe evap-orationofvolatilecompoundsfromthefilm[13].Pigmentsmade fromreactivemetalsactascatalysts,whichacceleratedrying.This isthecase,forinstance,ofcobalt-basedpigment.

Cobalt Blue pigment, known as Pigment PB 28, is a cobalt (II) oxide-aluminium oxide, or cobalt (II) aluminate, CoAl2O4 madebysinteringcobalt(II)oxidewithaluminaattemperatures above1200◦C.CobaltbluewassynthesizedbyThénard in1803 andimmediatelymanufacturedonacommercialscale;itrapidly becameonethemostcommonbluepigmentsusedinmodernart [16,17],despitebeingrelativelyexpensive.Thiscouldexplain even-tualcasesofpigmentadulteration(eitherwithorganicorinorganic pigments)throughouthistorysuchasthemixturewithartiﬁcial ultramarine(PB29,Na8-10Al6Si6O2-4S2-4)[18].Cobaltbluepigment isalsoasurfacesiccative.Itisthemostcommonlyusedprimary drierandthemostactive.Itactsasanoxidationcatalystsinceits exhibitsmorethanoneoxidationstate.Asotherprimarydriers, cobaltbluepigmentcatalyzescross-linkingofthemediumand

-https://doi.org/10.1016/j.culher.2018.12.007

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Table1

Commercialoilandalkydpaintsamplesstudied(fromMecklenburg’sPaintReferenceCollection).

Sample Manufacture

descriptiona

Thickness Castingdate Tensiletestsrun(different samplescutfromthesame pieceofcastpaint)in

Observations GR-CB-ARLO GRUMBACHER® ArtistsOil Colors Cobaltblue, PB28 Alkalireﬁned linseedoil 250␮m 1999 2001(Mecklenburgetal., 2013)[11] 2017(thisstudy) GA-CB-ARLO GAMBLIN® ArtistsOil Colours Cobaltblue, PB28 Alkalireﬁned linseedoil 250␮m 1999 2001(Mecklenburgetal., 2013)[11] 2017(thisstudy) GA-CB-LIT GAMBLIN® ArtistsOil Colours Cobaltblue, PB28

Linseedoil 230␮m 1999 2001(Mecklenburgetal., 2013)[11] 2017(thisstudy) Signiﬁcant wrinklesobserved onthesurface Darkertone WN-CB WINSOR& NEWTON® ArtistsOil Colours Cobaltblue, PB28 Linseed/safﬂower oils 340␮m 1998 2001(Mecklenburgetal., 2013)[11] Toosticky Itwasnotpossible totestitin2017 WN-ALK W&N® Alkyd Paint Cobaltblue, PB28

Alkydresin 200␮m 1998 2001(Mecklenburgetal., 2013)[11] 2017(thisstudy) Microprotuberances onthesurface WN-GR-ALK W&N® Grifﬁn Artists’Alkyd Paint Cobaltblue, PB28

Alkyd 160␮m 1999 2001(Mecklenburgetal., 2013)[11]

2017(thisstudy) CB:cobaltblue;WN:Winsor&Newton®

;GA:Gamblin®

;GR:Grumbacher®

;LSO:linseed/safﬂoweroils;ARLO:alkalireﬁnedlinseedoil;LO:linseedoil;LIT:withthe commercialadditionofLitharge.

a_Based_on_{manufacturers’}_datasheets_available_in_the_{1980–1990’s.}

withinthisprocess-hydroxyl,carbonylandcarboxylicgroupsare formedacceleratingthedryingofthesurface.

ThisdoubleroleofCoAl2O4(asapigmentandasiccative)has evidentimplicationsintheconservationpractice,inparticularin theformationofsuperficialwrinkles.Thepresenceofsurface wrin-klesinpaintfilmshastraditionallybeenexplainedasaconsequence oftheplasticflowexperiencedbythepaintfilmduetotheexcess ofbinderandlowpigmentvolumeconcentration(PVC),and there-foreusuallyassociatedtolongdryingtimes.Nevertheless,thereare othersourcesforthewrinkledappearanceofcobaltbluepaintfilms suchasitssiccativenature.Beingcobaltblueasurfacesiccative,it tendstocreateaninitialsurfacefilmthatslowsdownthediffusion ofoxygeninthebulkofthepaint,leadingoftentoadistinctiveeffect intheformofwrinkles(seefiguresinTables1and2).Inthislatter case,wrinklesoccurnaturallyasaconsequenceofpigment-binder interactionduringthedryingprocessandwherePVCnot necessar-ilyhasanyinfluence.Bothmechanismsarechallengingbutrequire adifferentapproachinday-to-dayconservationpractice.

1.2. Aimofthisstudy

Inthispaper,themechanicalpropertiesofselectedcommercial oilandalkydpaintscontainingcobaltbluepigmentarestudied.For thispurpose,manufacturedformulationsandcustom-madepaint reconstructionswereprepared.Theaimofthisstudywastoassess thedifferentmechanicalbehaviourofthepaintfilmstestedandto gainaninsightintocomplexpaintformulations.Theinfluenceof someprimaryfactorssuchastheamountandtypeofpigmentand binder,thepresenceandtypeofadditivesaswellasthehydrolysis mechanismsexperiencedmayhaveinthelong-termmechanical behaviourofthepaintfilmsstudiedisdiscussed.

2. Materialsandmethods

2.1. Paintsamplesandsamplepreparation

Inthisresearch,threedifferentsetsofcobaltbluepaintﬁlms werestudied.

We selected four cobalt blue oil samples manufactured by Grumbacher,GamblinandWinsor&Newton(W&N)andtwocobalt bluealkydoilpaintfilmsfromW&NbelongingtotheMecklenburg’s PaintReferenceCollectionattheSmithsonianMuseum Conserva-tionInstitute(USA).Theoilfilmstestedanddiscussedinthispaper werepreparedlateinthe1990’sonpolyesterfilmandallowedto dryincontrolledenvironmentalconditionssincethen(Table1).

A second set of samplesconsists of four commercialcobalt blueoilpaintsmanufacturedbyTitan,W&N,Talens-VanGoghand MaimeriasreportedinTable2.Thepaintswerepurchasedand castin2014onpolyester ﬁlmandallowedtodryincontrolled environmentalconditions.

Thethirdsetofsamplesconsistsoffivecustom-madepaintfilm reconstructions,(Table3).1LwaspreparedmixingcobaltbluePB28 andcoldpressedlinseedoil(CPLO)fromKremer.2L,3Land4Lwere preparedsimilarlybutwiththeprogressiveincorporationof addi-tives:25%ofthetotalamountofpigmentwereinorganicfillers (mixtureofcalcite,gypsumandkaolin).Fivepercentaluminum stearateand10%castorwax(i.e.hydrogenatedcastoroilpurchased fromSigma-Aldrich)werealsoaddedtogainaninsightintothe effectsspecificadditivescaninduceinthemechanicalproperties ofagivencobaltblueoilpaintfilm.Thepaintfilmlabelledas5F waspreparedbymixingCPLOwithsunflowerandsaffloweroils (Sigma-Aldrich)togetherwithalltheabove-mentionedadditives. Thisformulationisconsistentwiththecomplexmixturesoforganic

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Table2

Commercialoilpaintsamplesstudied(2014).

Sample Manufacture

description

Thickness Castingdate Tensiletestsrunin Observations

WN-CB178 W&N® Artists’ Paint Cobaltblue, PB28 Linseed/ safﬂoweroils 165␮m 2014 2016(thisstudy) MA-CB Maimeri® Artisti374 Cobaltblue, PB28

Oil 132␮m 2014 2016(thisstudy) Toofragiletobe tested TI-CB Titan® Coloral óleoextraﬁno 52 Cobaltblue, PB28 +PB29

Oil 350␮m 2014 2016(thisstudy)

TA-CB Talens-Van Gogh® Oil Colour511 Cobaltblue, PB28

Oil 90␮m 2014 2016(thisstudy)

Table3

Representativecustom-madecobaltoilpaintﬁlms(2014).

Sample Description Thickness Castingdate Tensiletestsrunin

1.L CPLO (54.5%)

CobaltbluePB28 (45.5%)

Noadditive 250␮m 2014 2017(thisstudy)

2.L CPLO (50%)

CobaltbluePB28 (50%)

CaCO3+CaSO4+2SiO2·Al2O3·2(OH2)(25%

ofpigment) 150␮m 2014 2017(Thisstudy) 3.L CPLO (50%) CobaltbluePB28 (50%)

ofpigment)+[CH3(CH2)16COO]3Al(5%of pigment) 140␮m 2014 2017(thisstudy) 4.L CPLO (50%) CobaltbluePB28 (50%)

ofpigment)+[CH3(CH2)16COO]3Al(5%of

pigment)+castorwax(10%ofpigment)

110␮m 2014 2017(thisstudy) 5.F CPLO+ safﬂower+ sunﬂower (50%) CobaltbluePB28 (50%)

ofpigment)+[CH3(CH2)16COO]3Al(5%of

pigment)+castorwax(5%ofpigment)

190␮m 2014 2017(thisstudy)

andinorganiccompoundsthatcanbefoundinmanufacturedpaints

[3,19].

2.2. Analyticaltechniquesandmethods

Theselectedpaintﬁlmswerestudiedbythejointuseof sev-eralanalyticaltechniquestoobtaindataabouttheircomposition, morphologyandmechanicalbehaviour.

Tensile tests were conducted using custom built miniature tensiletesters[21].Averagesamplemeasurementswere6.5mm (width)×0.30mm(thickness)and7.5mm(gaugelength). Aver-agethickness and widthwere measuredthree timesalong the gaugelengthwithboth a Mitutoyo103–137 micrometerand a digital callipergauge micrometer.Two samplesfrom thesame paintstripscastinthe1990sandtested byMecklenburginthe veryearlystagesoftheirdryingprocess[20,21]weretestedfor thisstudythusallowingthemonitoringofchangesinducedafter havingbeendryingfor18yearsinacontrolledenvironment.For thispurpose,thepolyesterﬁlmwasremovedandthetestswere runonfreeunsupportedpaintﬁlms.Thesampleswereclamped

in thetesting gauges and allowedtocome into equilibrium in thechamberfor48hat50±0.5%RHand23±0.5◦Cpriorto test-ing.ATesto608-H2thermohygrometertogetherwithaprecision hygrometer(MitchellOptidew VisionHighPerformance Optical Dew-PointTransmitter)monitoredrelativehumidityand tempera-tureinthechamber.Allthetestswereconductedatthesamestrain rate.Incrementsof0.0625mmstrainwereappliedprogressivelyat 30-secondintervals.Testwereperformedat3.75mm/minspeed. Load-displacementdatawereconvertedtostressandstrainvalues usingacustomizedMicrosoftExcelspreadsheet.

Microsamplesoffreefilmsweretakenfromthedriedpaints andobservedwithaLeicaDMRmicroscopeinreflectedlight(25× to400×magnification).

AJeolJSM6300scanningelectronmicroscopeoperatingwith aLink-Oxford-Isisenergy-dispersiveX-raymicroanalysissystem was used to observe surface and cross sections of the ﬁlms (20kV accelerating voltage, 2.10–9A beam current and 15mm distance).Elementalsemi-quantitativeweightpercentageswere calculated by EDX probe according to ZAF matrix correction [22–24].

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Table4

Resumeoftheresultsofthechemicalcharacterizationofthecommercialoilandalkydpaintsamplesstudied(fromMecklenburg’sPaintReferenceCollection). Samples Optical

microscopy

SEM observations

EDX VIS-reﬂectance FTIR-ATR(D2nd) GC-MS Considerations GR-CB-ARLO Lowroughness

Lightblue Cross-section: compactﬁlm. Darktoneonthe bottomand lightercloseto thesurface

Fine granulometry withsomecoarse grains Organicmaterial concentratedon thesurface Homogeneous distributionofAl andCo

Co,Al,Ca,P,Si, Mg

%Co=21

Cobaltblue Dryingoil Carboxylatesand freefattyacids (stearates) CoAl2O4

Silicates Calciumsulphate

Saturatedmonocarboxylic (lauric,myristic,palmitic, stearic)anddicarboxylic (suberic,azelaic,sebacic)acids; unsaturated(oleic)

Acids;glycerol P/S=0.8 A/P=2.5 O/S«0.1 GA-CB-ARLO Plainandglossy

surface Lightblue Cross-section: compactﬁlmand uniformtome Homogeneous andcompact granulometry (wellgrinded: 1␮m) Uniform distributionofCo andAlparticles

Co,Al,Ca,Si %Co=24

Cobaltblue Dryingoil Carboxylatesand freefattyacids (calcium sterate?) Calcium carbonate Calciumsulphate CoAl2O4 Saturatedmonocarboxylic (lauric,myristic,palmitic, stearic)anddicarboxylic (pimelic,suberic,azelaic, sebacic)acids;unsaturated (oleic)

Acids;glycerol P/S=1.1 A/P=2.3 O/S«0.1 GA-CB-LIT Darkblueand

opaquesurface Wrinklesonthe surface Cross-section: heterogeneous tone(lighterin themiddle) Homogeneous andﬁne granulometry: uniform distributionofCo andAlparticles +Pbparticles

Co,Al,Pb,S,Fe, Ca

%Co=14

Cobalt blue+litharge

Dryingoil(free fattyacidsand Carboxylates) CaCO3

CoAl2O4

Saturatedmonocarboxylic (lauric,myristic,palmitic, stearic)anddicarboxylic (pimelic,suberic,azelaic, sebacic)acids;unsaturated (oleic)

Acids;tracesofglycerol P/S=1.6 A/P=4.2 O/S«0.1 WN-CB Homogeneous surface Lightblue Cross-section: heterogeneous tone Very homogeneous andﬁne granulometry Concentrationof organicmaterial Uniform distributionofCo andAl

Co,Al,Ca,Mg,Si, K,S,Fe %Co=18

Cobaltblue Dryingoil Aluminium stearate (carboxylates) CoAl2O4 Silicates DOLOMITE Saturatedmonocarboxylic (lauric,myristic,palmitic, stearic)anddicarboxylic (pimelic,suberic,azelaic, sebacic)acids;unsaturated (oleic)

Acids;glycerol;long-chain monocarboxylic(behenicand arachidic)acids

P/S=2.8 A/P=2.1 O/S=0.4 WN-GR-ALK Plainand

homogeneous surface Medium-light blue Cross-section: compactﬁlm Homogeneous andﬁne granulometry: verycompact Uniform distributionofCo andAl

Co,Al,Ca,Zn,Si, K,Mg,Fe %Co=24 Cobalt blue+white pigment (probablyzinc white) Alkydresin, carboxylates(Zn soaps) CoAl2O4 Silicates Saturatedmonocarboxylic (palmitic,stearic)and dicarboxylic(suberic,azelaic, sebacic)acids;unsaturated (oleic)

Acids;glycerol;

pentaerythritol;phtalicacid; 12-hydroxystearicacid P/S=1.6

A/P=1.9 O/S=0.1 WN-ALK Plainand

homogeneous surface Medium-light blue Cross-section: compactdraft and heterogeneous toneforlighter particles Presenceof surfacecracking Fine granulometry anduniform distributionofCo andAl

Co,Al,Ca,Fe,S, Mg,Zn %Co=21 Cobalt blue+white pigment (probablyzinc white) Alkydresin Carboxylates (Stearateof Aluminum) CoAl2O4 Calciumand magnesium carbonates Saturatedmonocarboxylic (palmitic,stearic)and dicarboxylic(pimelic,suberic, azelaic,sebacic)acids; unsaturated(oleic) Acids;glycerol;

pentaerythritol;phtalicacid; 12-hydroxystearicacid P/S=1.7 A/P=1.9 O/S«0.1 WN-CB178 Glossyand wrinkledsurface Medium-dark blue Cross-section: dividedintwo, darktoneonthe surface Presenceof surfacecracking Fine granulometry anduniform distributionofCo andAl Co,Al,Mg,Cl,Cr, K %Co=18

Cobaltblue Dryingoil (partially unsaturated) Carboxylates (aluminium stearate) CoAl2O4 Silicates Calciumsulphate Dolomite Saturatedmonocarboxylic (lauric,myristic,palmitic, stearic)anddicarboxylic (suberic,azelaic,sebacic)acids; unsaturated(oleic)

Acids;glycerol;long-chain monocarboxylic(behenicand arachidic)acids

P/S=2.5 A/P=1.4 O/S=1.3

Linseedoiland safﬂower/sunﬂower

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Table4(Continued) Samples Optical

microscopy

SEM observations

EDX VIS-reﬂectance FTIR-ATR(D2nd) GC-MS Considerations MA-CB Glossyand

wrinkledsurface Lightblue Cross-section: stratiﬁedsurface Fine granulometry andhomogenous Lighterparticles ofadditive Co-concentred onbulkandAlon thesurface

Al,Co,Si,Na,Zn, Cl

%Co=24

Cobaltblue Dryingoil (partially unsaturated) Carboxylates (aluminiumand zincstearate) CoAl2O4 (Green Phtalocianine?) Silicates Saturatedmonocarboxylic (lauric,myristic,palmitic, stearic)anddicarboxylic (suberic,azelaic,sebacic)acids; unsaturated(oleic)

Acids;glycerol P/S=1.9 A/P=2.1 O/S=0.5 TI-CB Glossysurface

Medium-dark tone Cross-section: compactand homogeneous draft Heterogeneous granulometry withbigger particlesofCa Surfacecracking Uniform distributionofCo andAl

Co,Al,Ca,Si,S, Na,K,Fe %Co=9 Cobalt blue+ultramarine Dryingoil (partially unsaturated) Carboxylates (aluminium stearate) CoAl2O4 Ultramarineblue Calciumsulphate Silicates Calcium carbonate Saturatedmonocarboxylic (lauric,myristic,palmitic, stearic)anddicarboxylic (suberic,azelaic,sebacic)acids; unsaturated(oleic)

Acids;glycerol P/S=1.8 A/P=1.4 O/S=1.3

TA-CB Satinsurface Darkand uniformtone Cross-section: heterogeneous tone(lighterin themiddle) Homogeneous granulometry andsuperﬁcial cracking Uniform distributionofCo andAl

Co,Al,Ca,Zn,Mg %Co=10

Cobaltblue Dryingoil (partially unsaturated) Carboxylates (zincstearate) CoAl2O4 Silicates Calcium carbonate Saturatedmonocarboxylic (lauric,myristic,palmitic, stearic)anddicarboxylic (suberic,azelaic,sebacic)acids; unsaturated(oleic)

Acids;glycerol P/S=0.9 A/P=0.7 O/S=1.8

Fig.1. a:stressversusstrainplotsofcobaltbluecommercialoilpaintﬁlmstestedin1998–99;b:stressversusstrainplotsofcobaltbluecommercialoilpaintﬁlmstested in2017.

FTIR-ATR analyses were performed with a Thermo Nicolet

Nexus670FTIRspectrophotometercombinedwithaSmartOrbit

SingleReﬂectionDiamondATRaccessory,from4000to400cm−1 for128scanswith4cm−1resolution.Spectrawereelaboratedwith Omnic8.0.intherange1800–1600cm−1,thesecondderivativewas calculated.

ATraceGC2000instrumentwithacapillarySupelcoEquity-5

column,30m,0.25mm,0.5␮minterfacedwithaTraceMS2000

wasemployedforGC-MSanalysis.Thetemperatureprogramme

wassetfrom80◦Cto300◦C 10◦C/min.TheMSwasruninFull

Scan mode (m/z40–600), 1.9 scans/s.The transfer line wasat

280◦Cand thesourcetemperature was150◦C. Electron ionisa-tionenergywas70eV.QuantitativeGC-MSanalysiswasperformed usingnonadecanoicacidastheinternalstandard.Theoilsamples weretransestherifiedusing(trifluoromethylphenyl) trimethylam-moniumhydroxide,overnightreactionasdescribedin[2,16].The molarratiosamongstthemostsignificantfattyacidswere calcu-lated:palmitictostearicacid(P/S),azelaictopalmiticacid(A/P),

oleictostearicacid(O/S)ratioswereconsideredandcomparedto previousresultsobtainedbytheauthorson20thcenturyartistsoil paintsasdescribedin[2,3,25].

3. Resultsanddiscussion

Table 4 reports a detailed summary of the multi-analytical chemical-physicalresultsonthestudiedpaintﬁlms.

Althoughtheselectedpaintshavesimilarcommercialnames andweresoldas“cobaltblue”paints,theirsurfaceappearanceis quitedifferent(seepicturesinFigs.1and2)andtheirformulations arerathercomplex.

Theanalyticalresultscanbebrieﬂysummarisedhighlighting: • theuseofdifferentbindingmedia(foroilpaints:rangingfrom

traditionallinseedoiltolessdryingoils,suchassafﬂowerand sunﬂoweroils;foralkyds:alkydresinanddryingoils);

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Fig.2. a:chromatogramGR-CB-ARLO;b:chromatogramWN-CB-ALK.

Fig.3.Stressversusstrainplotsofcobaltbluecommercialoilpaintﬁlmscastin2014.

• theadditionof inorganicand organicpigmentstocobaltblue (suchas,forinstance,ultramarineblue);

• variablepercentagesofcobalt;

• thepresenceofextendersandﬁllers(suchassilicates,calcium sulphate, calcium carbonate, dolomite,etc) dispersing agents (suchasmetalstearates)andotheradditives(zincwhite).

Theseﬁndingsareconsistentwiththeliteratureaboutthe anal-ysisofmodernandcontemporaryoil-basedpaints[1–6,15,19].

Asconcernsthemechanicalmeasurements,notallthepaints weretested.Freeﬁlmswereobtainedandtestedforallsamples exceptforWN-CB(fromthe1990s),Talens-VanGoghandMaimeri (castedin2014),becauseitwasnotpossibletodetachthepaint ﬁlmsfromthepolyestersubstrate.

3.1. Commercialcobaltblueoilpaintﬁlms

Fig.1apresentsthestress-straincurvesofGR-CB-ARLOand GA-CB-ARLOpaintsamplestestedbyMecklenburgetal.after2years drying[21].BothGamblinandGrumbacherpaintfilmspresenta weakbehaviourwithverylittlestrength(lessthan0.5MPa)and1% elongation.Thelowmodulusofelasticityobservedcouldindicatea veryyoung(andnotcompletelycross-linked)oilpaintfilm.Fig.1b showsthemechanicalpropertiesofthesamepaintfilmsafter18 yearsdrying.Surprisingly,theypresentthesameweakbehaviour (lessthan0.5MPa)depictedinFig.1a,togetherwithasignificant increaseinplasticity(from0.01mm/mmstrainobservedin2001 inboth samplesto0.04mm/mm and0.05mm/mmobserved in 2017forGrumbacherand Gamblin,respectively).In bothcases, onecouldexpectstifferandstrongerpaintfilmsasaconsequence

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Fig.4.Secondderivativeofthe1800–1600cm−1_range_(sample_WN-CB_178).

oflongerdryingtimesandthereforeamorecrosslinkedstructure, asusuallyobservedinoilpaints.Nevertheless,theincreased plas-ticitycouldbeaconsequenceoftheearlyhydrolysisexperienced bytheoilpaintﬁlm[26].

GA-CB-LIT(aGamblinpaintsamplecontaininglitharge,PbO, andcastin1999)wastestedforcomparisonpurpose.Previous stud-ies[12,13,20]shownthat,eveninsmallamounts(upto1.6%),PbO isaneffectivedrierforoils(linseed,poppyseed,walnutand saf-flower)traditionallyusedbyartists.Itcanspeedupthedryingof oil-basedpaintsandinducesignificantdifferencesintheir mechan-icalproperties.Nevertheless,thisdoesnotnecessarilymeanthatit contributestoformbetterpaintfilms.

In this study,it wasinteresting tocompare cobaltblue and lithargesiccative properties.Evenifcobalt hasalready relevant siccativeproperties,lithargeseemstoenhancethem.Resultshave shownthatthepresenceofalowamountoflithargepromotesthe thoroughdryingofGA-CB-LITinthebulkbutalsointhesurface. Thiscouldalsoberelatedtoitswrinkledappearance.

Consideringthelipidicbindingmedia,itisinterestingthat Gam-blinandGrumbacherwerefoundtocontainalkalirefinedlinseed oil(i.e.arawlinseedoiltreatedwithalkalitoreducethefree acid-itybyformation ofwater-solublesalts,which aresubsequently removedbywashing).Aspreviouslystated,differentdryingspeeds canbeobservedasafunctionoftherefiningprocessoflinseedoil: therefininginalkalioracidicmediumsmakeslinseedoildrytwo timesfasterthancoldpressedoils[12,13].Moreover,thelowP/S ratiosofGrumbacherandGamblin(0.8and1.1,respectively)could berelatedtotheadditionofdispersingagents(suchasaluminium stearates),asreportedinpreviousstudies[2,3,15,19].Thesemetal soapsaregenerallyusedtoraisetheacidityoftheoilandtowet pig-ments,inparticularwhenaddedtoformulationscontainingalkali refinedlinseedoilswithareducedamountoffreefattyacids[27]. Fromamechanicalpointofview,theirpresencecouldbe respon-siblefortheincreaseinplasticityobservedwithageinsamples GR-CB-ARLOandGA-CB-ARLO[1,2,27].

AnotherconsiderationarisesobservingtheA/Pratiosfor GR-CB-ARLO,GA-CB-ARLOandGA-CB-LIT(2.5,2.3,and4.2,respectively). HighA/Pratios(higherinGA-CB-LITduetothecatalyticactionof leadions) areusuallyrelatedtoa highdegreeofchain scission whichcouldalsoexplainalesscrosslinked(andthereforemore ﬂexible)paintﬁlm[28](seeFig.2).

Fig.3representsthestress-straincurvesofthreecobaltblue Titan,W&NandTalens-VanGoghcommercialoilpaintﬁlmscast

in2014(correspondingtosamplesTI-CB,WN-CB178andTA-CB, respectively),andtestedafter3yearsdrying.Ashighlightedbythe curves,thesamplesshowsigniﬁcantdifferencesintheir mechani-calproperties.

TheﬁrstaspectthatdeservesattentionistheUTS:W&Nisabout morethan twotimesstronger (2.5MPa)than Titanand Talens-VanGogh(1MPaand0.7MPa,respectively).

ApossibleexplanationislinkedtotheGC-MS resultsonthe organicfraction.Talens-VanGogh,MaimeriandTitanpaintfilms werefoundtobeboundinlinseedoil,sincetheirP/Sratiosare con-sistentwithlinseedoilvalues(Table4).However,theP/Sratiofor W&Npaintis2.5and,togetherwitharelevantpresenceof spe-cificfattyacids(arachidicbehenicacids)initscomposition,may indicatethepresenceofsafflowerand/orsunfloweroilinthepaint formulation[3,15].AsimilarP/Sratio(2.8)wasfoundinWN-CB, castin1998,whichisinaccordancewiththeinformationprovided inW&N’sdatasheetsinwhichamixtureoflinseedandsafflower oilsisdeclared.

Theuseofslowdryingoilssuchassafflowerand/orsunflower oilinpaintsdecreasethespeedofdrying.Arelativelyyoungpaint filmcontainingsafflowerand/orsunfloweroilswouldbeexpected to remain flexible (even plastic) during a long period of time. Nevertheless,thesteppercurveobtainedforsamplesWN-CB178 deservesfurtherattention.AsreportedinFTIR-ATRspectrumof Fig.4,thepresenceofdolomitewasdetectedwhichisan indica-tionofthepresenceoffillers.Ahighpigmentvolumeconcentration could have counteracted the action of the slow drying oil by speedingupitsdryingaswellasthestiffeningofthepaintfilm [15].

Talens-VanGogh,beingapparentlystiffer,appearedtobe sig-nificantlyweakerthanW&NandTitan filmsfrom2014(Fig.3). AspreviouslyobservedinGR-CBandGA-CBfilms,theP/Sratio<1 ofTalens-VanGoghcouldsuggesttheadditionofmetalstearates, visiblealsobyFTIR.EDX elementalanalysishashighlightedthe presenceofZinc,likelyintheformofzincoxide(orzincwhite), whichisfrequentlyaddedincommercialpaintformulationsasa whitener/extender.Inallthreemock-ups,itisrelevanttonotethat, havingbeendryingforonly3years,theirUTSissignificantlyhigher (2.5MPaforW&Nand1MPainthecaseof Titan)thantheUTS observedinthesamplesbelongingtotheMecklenburg’scollection testedafter18years(∼0.35MPa).ThepresenceofZnOwhitemakes paintfilmsbecomebrittleintheirearlydryingstagesbutalsoto causedelaminationproblems.Zinc,beinganothermetalcompound

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Fig.5. a:SEMMA-CB:CoandAlparticlesdistribution;b:SEMGA-CB-ARLO:CoandAlparticlesdistribution.

existinginmorethanonevalencestate,speedsupthedryingrate ofdryingoils[29–31].

Regardingtheroleofcobaltbluepigmentinthemechanical per-formanceoftheselectedoilpaintfilms,thereareseveralaspects tobehighlighted.CobaltBluewasidentifiedinallthecommercial samplesstudied(Table4)asindicatedbythetypicalIRabsorptions ofcobaltoxide(c.a.640,543andasidekickat800cm−1)andof alu-miniumoxide(486cm−1)andtheVIS-Reflectancespectrashowing thetypicalweakinflectionat480nmandthreesub-bandsat548, 583,and627nm[32,33].InthecaseofTitan,ultramarinebluewas detectedaswell.Thisisinterestingsinceitisknownthat–asit happenswithcobalt–,ultramarinehasaveryhighoilindex pig-mentandthereforeahighoilcontent.Cobaltbluepigmentpresents averyhighoil-absorptionindex,whichmeansthatitneedsa sig-nificantamountofoiltoformauniformpaintfilm[16].Thiscanbe relatedtotheactivityofmetalcations.Sincethecatalyticactivity duringoxidativepolymerizationfallswitha decreasing propor-tionalcontentofcobaltcationsdissolvedintheorganicphase,itcan

beexpectedthatcobaltoilpaintﬁlms,beinghighlyabsorbent,will experienceslowdryingtimes.Thepresenceofahigheroilcontent couldexplainthelowerE-modulusandhigherﬂexibilityobserved forTI-CB[16,34].

SEManalysisevidencedauniformdistributionofcobaltand alu-miniumparticlesexceptforMA-CB,wherecobaltparticlesseemto bemoreconcentratedonthebulk,whereasaluminiumparticles tendtoconcentrateonsurface(Fig.5).EDXanalysisrevealedan extremelyvariable%Coprobablyduetothepresenceofother com-poundsinthedifferentpaintmatrixes(Table4).Inaddition,both thesizeandshapeofpigmentparticlesaswellasthechemical com-positionofothermineralspresentinthematrixmightcontribute tomakethecatalyticactivitydecrease. ˇSim ˚unkováetal.suggest thattherateofpolymerizationofthedryingoilincreaseswithPVC anditisdifferentforthevarioustypesofcobaltbluepigment[35]. OtherstudiesrefertotheinfluenceofPVCinthemechanical prop-ertiesofpaintfilms.Forexample,ahighPVCwillmakepaintfilms bestifferandhavealowerfailurestrain.Nevertheless,abovethe

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Fig.6. a,b,c:stressversusstrainplotsofcobaltbluecommercialalkydpaintﬁlmscastinthelate1990’sindifferentdryingstages.

Fig.7. Stressversusstrainplotsofcobaltbluerepresentativecustom-madesamples.

CriticalPigmentVolumeConcentration(CPVC),paintﬁlmspresent alowUTSandfailurestrainbutalsoapoorlyboundpigment[36]. 3.2. Commercialcobaltbluealkydpaintsﬁlms

Fig.6ashowsthestress-straincurveofW&NAlkydPaintand W&NGriffinArtists’AlkydPaint(samplesWN-ALKandWN-GR-ALK respectively).Cobaltbluealkydcommercialpaintstestedinthis studyaremorethantentimesstronger,stifferandelongablethan commercialcobaltblueoilpaintsstudiedaftersimilardryingtimes. Alkydresinsareoil-modifiedalkydpolyestersor,inotherwords, artificialdryingoilsthatautoxidizeandcross-linkdependingon theirdegreeofunsaturation[5].AccordingtotheGC-MSresults, WN-ALKandWN-GR-ALKcontainanalkyd resinmodified with linseedoil(P/S=1.6and1.7,respectively)andhydrogenatedcastor oil(alsoknownascastorwax)whosemarker–12-hydroxystearic acid–,wasfoundinbothsamples[37].Thiswax-likematerialis oftenaddedinoilandalkyd-basedpaintformulationsasa stabi-lizerorrheologyadditivetopreventoilseparation.Thepresence ofwaxymaterialsasstabilizersisknowntoincreasethe wettabil-ityofthepigment,asasortoflubricant,increasingoilabsorption [34,35].Thiscouldexplainthehighfailurestrainsobserved.

Anotherrelevantaspectisthepresenceofzincwhiteinboth samples(Table4).Asexplainedpreviously,zincionsaresufﬁcient mobiletointeractwithcobaltandmake thepaintﬁlmbecome stifferinarelativelyshortperiodoftime[14] asFig.6ashows. Fig.6bshowstheevolvingmechanicalpropertiesofWN-ALKin dif-ferentdryingstages(after2,8and18years).Asitcanbeobserved, thesamplebecomesstifferandstrongerovertimeandthesame appliesforWN-GR-ALK(Fig.6c).

3.3. Cobaltbluecustom-madepaintﬁlmreconstructions

Aseriesofrepresentativemock-upsampleswerepreparedin 2014(Table3)aimingatgaininganinsightintotheroleeach com-ponentplaysinthemechanicalbehaviourofagivenoilpaintﬁlm. Fig.7ashowsthestress-straincurveofthesamples1L,2L,3L,4L and5F.Inthecaseof1L,2L,3Land4Ladetailedlookattheelastic regionofthedifferentstress-straincurves(Fig.7b)suggestthat2L (thesamplewiththeadditionofﬁllers)istheonewitha highE-Modulus.Eveniffurthertestsareneeded,thisincreasedstiffness

couldberelatedtothefactthattheadditionofﬁllers(untilacertain CVPV)canmodifythePVCofthesampleformulatedwithbinder andpigmentaloneaspreviouslyexplained.

Theadditionofstearates (3L)showsacurvewithalower E-ModulusbutthatreachesahigherUTSthan2L,whereas4L(sample containing stearates and 10% castor wax) is more ﬂexible and presentsalowerUTS.Itcanbeobservedthat2L,3L,4Lpresent aconsiderabledeformationatbreak(0.15mm/mm)ifcompared withcommercialoilstested(0.05mm/mm)andcouldbeduetothe factthattherepresentativesampleswerefreshoilstestedshortly afterbeingcast(2years).

Finally,Sample5FcontainingamixtureofCPLO,safflowerand sunfloweroilsistwiceelongablethanthesamplesmadewithonly CPLO(1Lto4L),whichisinagreementwithpreviousstudiesabout theuseofslowdryingoilsandthereforethesignificantplasticity theyusuallypresent.Thisisparticularlytrueifwecompare5Fwith 4L,whichwaspreparedwithasimilaramountofpigment,fillers andadditivesbutwithadifferentmedium

4. Conclusionsandfutureperspectives

Thisstudyprovidesaninsightintothemechanicalproperties ofselectedcobaltbluepaintformulations.Besidesthecomplexity ofdefiningthecompositionofmodernoilandalkydpaintfilms labelledas“cobaltblue”paints,thereareoftenencountered syn-ergiccircumstancesthatmakeitdifficulttodiscriminateasingle causeforthechangesobservedintheirmechanicalperformance overtime.

For theoilpaintfilmsstudied,theinfluenceof thedifferent degreeofunsaturationofthelipidbindersinthemechanical prop-ertiesofthepaintfilmswasshown.Alkydpaintstestedwerefound tobemorethantentimesstrongerandstifferthanthecommercial cobaltblueoilpaintsstudiedaftersimilardryingtimes.Theyalso showedahigherelongationatbreak.Thiscouldbegovernedbythe differentbindingmediumbutalsobythepresenceofzincwhite, thatmighthavecontributedtomakethealkydpaintfilmbecome stifferinarelativelyshortperiodoftime.

Theamountofcobaltpresentmostlyaffectsitsdegreeof dis-persioninthelipidbinder.Thispointsoutthatagivencompound –either acting aspigment or as a siccative– might induce dif-ferent mechanical properties in the resulting paint ﬁlm as the

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stress-straincurvesobtainedfromtheexaminedcommercial for-mulationshaveshown.Ithasbeenobservedthatagreaterquantity ofpigmentabsorbedbythedryingoilcorrespondstoamore impor-tantcatalyticactivity(i.e.agreaterdryingspeedwithaconsequent increase inthe stiffness ofthe paintfilm).Furthermore,it was alsoobservedthattheadditionofotherdryingagentsmodifiesthe activityofcobalt.Asignificantexampleislithargethatcompetes withcobaltactingmainlyinthebulkofthepaintcreatingastiffer filmthatoxidizesfaster.Nevertheless,resultsshowedinthispaper makeitdifficulttoconfirmtowhatextentthequantityofcobalt influencestheconditionofthepaintfilmsexamined.

Asobservedinthecustom-madecobaltoilpaintfilmsstudied, additiveshaveshown toplay asignificantrolein the mechani-calpropertiesofthepaintfilmsstudied.Forexample,asignificant amountofdispersingagentswasfoundinthestudiedcommercial formulationsprobablyaddedtoslowdownthespeedofthedrying process.

Furtherresearchisneededtogainaninsightintothe relation-shipbetweenUTSthedryingoftheﬁlms(forexamplethroughthe evaluationoftheA/Pratios)andintotheroleofadditivesinthe mechanicalperformanceofcommercialpaintﬁlms.

Acknowledgements

The authors are deeply indebted to Dr. M.F.Mecklenburg (MuseumConservationInstitute-SmithsonianInstitution,USA)for providingthesamplestestedinthisstudyandfortheequipment donated.Thisstudywascarriedoutaspartoftheresearchproject HAR2016-75131-PgrantedbytheMinsteriodeEconomia,Industria yCompetitividad(MICINN/FEDER)andwasalsopossiblethanksto theﬁnancialsupportfromCa’FoscariUniversityofVenice(Premio GiovaniRicercatori).

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